A spark plug electrode for use in internal combustion engine

Abstract: In a spark plug electrode in which a noble metal alloyed tip is secured to a firing end of one of opposed electrodes which forms a spark gap therebetween, the alloyed tip has a noble metal component varied in an axial direction of the electrode depending on a total magnitude of spark discharges to which a specified portion of the tip is subjected between the opposed electrodes.

Description: The invention relates to a spark plug electrode in which a spark-erosion resistant noble metal containing tip is secured to a firing end of a centre or ground electrode. With the recent demand of enhancing a spark-erosion resistant property of an electrode for use in an internal combustion engine, Japanese Patent Publication No 62-31797 introduces a technique in which annular noble metal tips 103, 104 in turn cover firing ends of centre electrode metals 101, 102 of multi-polarity type and semi-creeping discharge type spark plugs 100, 100a as shown in Figures 11 and 12 of the attached drawings. JP-A-50 67488 discloses a similar spark plug electrode in which a precious metal chip, of a sintered body of a rare earth metallic oxide and iridium or ruthenium, is welded onto the centre eIectrode. US-A-4 963 112 discloses a spark plug electrode in accordance with the precharacterising portion of claim 1. Again, a platinum alloy chip is attached to the end of a centre electrode and laser welded on in such a way as to create an alloy zone between the chip and electrode of gradually decreasing platinum content away from the tip. With the arrangement illustrated in Figures 11 and 12, though, due to the positional relationship between the centre electrode metals 101, 102 and ground electrode 105, 106, the tips 103, 104 tend to be locally eroded in a manner as shown at a left half in Figures 11 and 12. Although the spark erosion makes spark gaps 107, 108 unacceptably greater which deteriorates the service life of the spark plug, an expensive noble metal component of the tip partially remains uneroded which makes the technique uneconomical. In order to avoid the uneconomical disadvantage, it has been considered to previously omit noble metal from an uneroded part of the tip as shown at the left half 109 in Figure 13a. However, the centre electrode metal 101 is spark eroded significantly as spark discharges repeatedly occur between the electrodes as shown in Figures 13b to 13d. This makes the service life of the spark plug 110 increasingly shorter than that of the spark plugs 100, 100a. This is presumably because the spark discharges still occur in the portion 109 of the electrode 101 where there is no noble metal, although the total magnitude of these spark discharges is substantially limited when compared with the portion including the noble metal. Therefore, it is an object of the invention to provide a spark plug electrode for use in an internal combustion engine which is capable of controlling a rapid spark erosion of an electrode to which a limited magnitude of spark discharges is subjected while maintaining an economical advantage by substantially reducing the quantity of noble metal component used to a portion of the electrode to which the limited magnitude of spark discharges is subjected. According to the invention, there is provided a spark plug electrode having an alloyed tip secured to a firing end of at least one of the opposed electrodes to form a spark gap between the opposing surface of said tip and opposed electrode respectively, the tip including a spark-erosion resistant noble metal, the noble metal content of the tip varying in an axial direction of said one electrode, characterised in that the noble metal content varies in a direction parallel to said opposing surface of the tip in dependency on the amount of sparking to which that portion of the tip is to be subject such that a largest weight percentage of the noble metal content is provided at portions of the tip subject to a relatively high amount of sparking, and wherein a smallest weight percentage of the noble metal content is provided at portions of the tip subject to a relatively low amount of sparking. Thus, the alloyed tip has a noble metal component varies in an axial direction of the electrode depending on the amount of sparking (e,g, magnitude of spark discharges or quantity of spark erosion), to which that portion of the tip is subjected between the opposed electrodes. Thus, the invention provides a spark plug electrode improved so that a noble metal component of the tip is altered depending on the amount of sparking to which a specified portion of the tip is subjected. The difference between an upper limit and lower limit of the noble metal component of the alloyed tip may be about 10% by weight or more. The electrode may have a clad metal and a heat-conductive core concentrically embedded in the clad metal. The clad metal may be a corrosion- and erosion-resistant nickel alloy containing 15 wt% Cr and 8 wt% Fe, and the heat-conductive core may be silver or copper. With the alloyed tip having the noble metal component varied in the axial direction of the electrode depending on a magnitude of spark discharges to which the specified portion of the tip is subjected, it is possible to increase the noble metal component of the tip to the portion to which increasing spark discharges or erosion are subjected. Further, it is also possible to increase the noble metal component of the tip to the portion to which the limited spark discharges or erosion are subjected. This enables a reduction in the difference between a quantity of spark erosion to which a low noble metal component of the tip is subjected and that of the spark erosion to which a high noble metal component of the tip is subjected. This ensures a uniform quantity of spark erosion all through the alloyed tip secured to an entire area of the firing end of the electrode, and thus enables an extended service life of the spark plug with a minimum quantity of noble metal. The invention will be further described by way of example only with reference to the accompanying drawings, in which:- Figure 1 is a plan view of a lower portion of a circular discharge type spark plug according to a first embodiment of the invention, but its left half is sectioned; Fig. 2 is a graph showing a relationship between a platinum containing rate (wt %) and an axial position of a noble metal alloyed tip to which varied spark discharges are subjected; Figs. 3a ∼ 3d are sequential views showing how the noble metal alloyed tip is secured to a center electrode; Figs. 4a and 4b are longitudinal cross sectional views of the lower portion of the circular discharge type spark plug to show manners how the tip is spark eroded; Fig. 5 is a graph showing a relationship between a platinum containing rate (wt %) and an axial position of a noble metal alloyed tip to which varied spark discharges are subjected according to a second embodiment of the invention; Fig. 6 is a perspective view of a lower portion of a multi-polarity type spark plug according to a third embodiment of the invention; Fig. 7 is a perspective view of a lower portion of a semi-creeping discharge type spark plug according to a fourth embodiment of the invention; Fig. 8 is a longitudinal cross sectional view taken along the line VII-VII of Fig. 7; Fig. 9 is a graph showing a relationship between a platinum containing rate (wt %) and an axial position of a noble metal alloyed tip to which varied spark discharges are subjected according to the semi-creeping discharge type spark plug shown in Fig. 7; Fig. 10 is a longitudinal cross sectional view of a lower portion of a spark plug according to a fifth embodiment of the invention; Fig. 11 is a longitudinal cross sectional view of a lower portion of a prior spark plug to show how a noble metal tip is spark eroded; Fig. 12 is a longitudinal cross sectional view of a lower portion of another prior spark plug to show how a noble metal tip is spark eroded; and Figs. 13a ∼ 13d are sequential views showing how a center electrode is spark eroded according to the prior spark plug shown in Fig. 11. Referring first to Figs. 1 and 2 which show a lower portion of a circular discharge type spark plug 1 for use in an internal combustion engine according to a first embodiment of the invention, the spark plug 1 has a tubular insulator 2, a metallic shell 3 in which the insulator 2 is placed. From a lower end of the metallic shell 3, a ring-shaped ground electrode 4 is integrally extended. In a manner to be surrounded by an inner wall 4a of the ring-shaped ground electrode 4, a lower portion of a center electrode 5 is arranged which is concentrically placed in the insulator 2. The insulator 2 is made of a sintered ceramic body such as, for example, alumina (Al₂O₃) with its inner space as an axial bore 6. The insulator 2 is further engaged against a shouldered inner wall 3a of the metallic shell 3 through a packing 7. Meanwhile, the metallic shell 3 is made of an electrically conductive metal such as a low carbon steel or the like so as to form a housing of the circular discharge type spark plug 1. To an outer surface of the metallic shell 3, a male thread portion 8 is provided to secure the metallic shell 3 to a cylinder head (not shown) of the internal combustion engine. A gasket 10 is provided between barrel portion 9 of the metallic shell and the cylinder head so as to air-tightly seal a combustion chamber (not shown) of the internal combustion engine. The lower end of the ground electrode 4 is arranged such as to oppose that of the center electrode 5, and being terminated short of the combustion chamber. Between the inner wall 4a of the ground electrode 4 and an outer surface of the center electrode 4, there is provided a spark gap G (e.g. 1.0 mm). The electrode 5 has an elongated column metal 11 and a noble metal tip 13 secured to a firing end 12 of the center electrode 5 which establishes a spark discharge against the inner wall 4a of the ground electrode 4. The column metal 11 of the center electrode 5 has a clad metal 14 and a heat-conductive core 15 concentrically embedded in the clad metal 14. The clad metal 14 has a corrosion-and erosion-resistant nickel alloy containing 15.0 wt % Cr and 8.0 wt % Fe. The clad metal 14 is solidly supported in the insulator 2 with its lower end somewhat extended beyond the insulator 2. The core 15 is made of a heat-conductive metal such as silver, copper, copper-based alloy or the like. That is to say, an increased quantity of the platinum component of the tip 13 resides in a portion corresponding to an upper limit 16 of the firing end 12 of the column metal 11. A decreased quantity of the platinum component of the tip 13 resides in a portion corresponding to a lower limit 17 of the firing end 12 of the column metal 11. The platinum component of the tip 13 is adapted to gradually change between the portions corresponding to the upper and lower limits of the firing end 12 of the column metal 11. The greatest quantity of the platinum component of the tip 13 is approximately 85 % by weight, while the least quantity of the platinum component of the tip 13 is approximately 70 % by weight. This concludes that a difference between upper and lower limits of the platinum component is about 15 % by weight. The following are a method how the tip 13 is secured to the firing end 12 of the column metal 11 of the center electrode 5 (Figs. 3a ∼ 3c). An annular groove 18 is provided on the firing end 12 of the column metal 11 (2.5 mm in diameter) by means of milling or the like. The groove 18 is 0.6 mm in length and 0.15 mm in depth. A distance between the lower limit of the column metal 11 and a center of the groove 18 measures 1.5 mm. A noble metal wire 19 is made of platinum, and formed into an annular configuration, both ends of which meets in a manner to have a slit 20 therebetween. The noble metal wire 19 is circular in section and 0.3 mm in diameter. The noble metal wire 19 is arranged so that its volume is substantially the same as that of the groove 18. After fitting the wire 19 into the groove 18 of the column metal 11, By means of electrical resistance welding, the wire 19 is provisionally secured to the left side of the groove 18 which corresponds to the lower limit of the firing end 12 subjected to the increased incidence of the spark discharges. Then, laser beams (LB) is applied perpendicular to the center of the groove 18 with a laser spot as e.g. 1.4 mm in diameter as shown in Fig. 3b, the wire 19 is thermally fused into the column metal portion in which the groove 18 is located. In this instance, the column metal 11 is rotated around its axis at the speed of e.g. (5π/6) rad/sec, while at the same time, applying 48-round of the laser beams (LB) to all the length of the wire 19 to carry out a seam welding. Upon carrying out the seam welding, a laser welding machine is used to generate a pulse-type YAG laser with an underfocus as 10 mm from an outer surface to the center of the column metal 11. The YAG laser is used with an output and pulse width as 6.5 J and 2.0 m sec respectively. Instead of the YAG laser, CO₂ laser may be used. Any type of welding may be used including electron beam welding so long as they can thermally fuse the wire 19 into the column metal portion in which the groove 18 of the column metal 11. Upon securing the wire to the groove 18, it is observed that a leading end of the uncoiled wire 19 may be fitted into the groove 18, and the column metal 11 is rotated independent of the wire 19, while at the same time, applying the laser beams (LB) to the leading end of the wire 19. After applying the YAG laser welding to the noble metal wire 19, the noble metal tip 13 is provided on the firing end 12 in the form of an alloyed layer in which the nickel alloy component of the column metal 11 and the platinum component of the tip 13 are thermally fused as shown in Fig. 3c. The positioning of the wire in the groove, together with the controlling of the intensity of the laser beam is used to control the distribution of noble metal. In this case, the nickel content increases as the laser beam is intensified and decreases where the laser beam is moderated. In this instance, the platinum component of the alloyed layer gradually increases from the lower limit to the upper limit of the column metal 11 as previously shown in Figure 2. With the structure thus far described, a high voltage is intermittently applied across the electrodes 4, 5 with the circular discharge type spark plug 1 mounted on the cylinder head of the internal combustion engine. The high voltage repeatedly induces the spark discharge between the inner wall 4a of the ground electrode 4 and the noble metal tip 13 provided on the firing end 12 of the column metal 11. As previously shown in Figs. 2 and 4a, the platinum component of the tip 13 is increased in which the increased incidence of the spark discharges occurs, while the platinum component of the tip 13 is decreased in which the decreased incidence of the spark discharges occurs. Videlicet, an increased quantity of the platinum component of the tip 13 resides in a portion corresponding to an upper limit of the firing end 12 of the column metal 11. Conversely, a decreased quantity of the platinum component of the tip 13 resides in a portion corresponding to a lower limit of the firing end 12 of the column metal 11. With the repeated cycles of the spark discharges between the inner wall 4a of the ground electrode 4 and the noble metal tip 13 by extending the use of the circular discharge type spark plug 1, the repeated spark discharges cause to erode the inner wall 4a of the ground electrode 4 and the noble metal tip 13 provided on the firing end 12 of the column metal 11. The spark erosion of the tip 13 depends on the platinum component and the incidence of the spark discharges to which the tip 13 is subjected. This means that there is no significant difference between the spark erosion of the tip portion to which the increased spark discharge incidence and increased platinum component are subjected and the spark erosion of the tip portion to which the decreased spark discharge incidence and decreased platinum component are subjected. The eroded layer of the tip 13 is uniformly retained all through the firing end 12 of the column metal 11 when the circular discharge type spark plug 1 exhausts its service life. As described above, the tip 13 is uniformly eroded all through the firing end 12 of the column metal 11 when repeated spark discharge is induced between the inner wall 4a of the ground electrode 4 and the noble metal tip 13 provided on the firing end 12 of the column metal 11. This enables to reduce the quantity of the expensive platinum component used to the tip portion to which the decreased incidence of the spark discharges is subjected without unadvantageously losing the spark erosion resistant property. This results in attaining a long service life of the spark plug with a minimum use of the expensive platinum, thus significantly curtailing a manufacturing cost when reduced to mass production in industrial application. Further, as previously shown in Fig. 2, the greatest quantity of the platinum component of the tip 13 is approximately 85 % by weight, while the least quantity of the platinum component of the tip 13 is approximately 70 % by weight. This concludes that a diffference between upper and lower limits of the platinum component is about 15 % by weight. This maintains a good spark erosion resistant property of the tip portion to which the decreased incidence of the spark discharges is subjected. It is possible to satisfactorily retain the above advantages by insuring the difference of 15 % by weight between the upper and lower limits of the platinum component of the tip 13. In addition, the platinum component of the tip 13 changes in the axial direction of the column metal 11, and thus dispersing thermal stress which would otherwise work locally on the tip 13 due to repeated heat-and-cool cycles while the circular discharge type spark plug 1 is in service. This arrangement also decreases the thermal expansional difference between the tip 13 and the column metal 11, and thus mitigating the thermal stress itself. This enables to effectively avoid cracks from developing on an interface between the tip 13 and the column metal 11, and preventing the tip 13 from inadvertently falling off the column metal 11. Fig. 5 is a graph showing how the platinum component changes depending on the position of the tip 13 according to a second embodiment of the invention. In this embodiment of the invention, an increased quantity of the platinum component of the tip 13 resides in a portion corresponding to an upper limit to the firing end 12 of the column metal 11. A decreasing quantity of the platinum component of the tip 13 resides in a portion corresponding to a lower limit of the firing end 12 of the column metal 11. The platinum component of the tip 13 is adapted to abruptly change between the portions corresponding to the upper and lower limits of the firing end 12 of the column metal 11. Again, this is achieved by controlling the intensity of the laser beam on welding. The greatest quantity of the platinum component of the tip 13 is approximately 83% by weight, while the least quantity of the platinum component of the tip 13 is approximately 71% by weight. This concludes that the difference between the upper and lower limits of the platinum component is about 12% by weight. This arrangement makes it possible to ensure the same advantages as obtained by the first embodiment of the invention since the difference between the upper and lower limits of the platinum component is 10% by weight or more. It is noted that the noble metal tip 13 may be used in a full creeping discharge type spark plug. Figure 6 shows a third embodiment of the invention in which a multi-polarity type spark plug 22 is used with paired ground electrodes 21 extended into the combustion chamber of the internal combustion engine. The noble metal tip 13 is welded to the column metal 11 of the center electrode 5 in the same manner as described at the first or second embodiment of the invention. Figs. 7 through 9 show a fourth embodiment of the invention which is applied to a semi-creeping discharge type spark plug 24. In this embodiment of the invention, a discharge gap (Ga) is provided which creeps between a front end surface 5a of the center electrode 5 and a discharge end 23a of a ground electrode 23 along a front end surface 2a of the insulator 2. An air gap (Gb) is provided between the discharge end 23a of the ground electrode 23 and an outer surface 3b of the insulator 3. The noble metal alloy tip 13 is made of a corrosion-and erosion-resistant platinum (Pt) or platinum alloy containing Ni and Ir, and formed into an annular configuration as shown in Figs. 3a ∼ 3c. With an experiment carried out to previously measure the incidence of spark discharges subjected to the tip 13, the noble metal component of the tip 13 is designed to change according to an axial position the elongated column metal 11 on the basis of the incidence of the spark discharges subjected to a specified portion of the tip 13. Consequently, the platinum component of the tip 13 increases in which the incidence of the spark discharges increases as shown at numeral 16 (the upper limit) in Fig. 2. Conversely, the platinum component of the tip 13 decreases in which the incidence of the spark discharges decreases as shown at numeral 17 (the lower limit) in Fig. 2. In the noble metal tip 13 welded to the firing end 12 of the column metal 11 in the semi-creeping discharge type spark plug 24, the platinum component of the tip 13 is increased in which the increased incidence of the spark discharges occurs, while the platinum component of the tip 13 is decreased in which the decreased incidence of the spark discharges occurs. Namely, an increased quantity of the platinum component of the tip 13 resides in a portion 25 corresponding to a central area of the firing end 12 of the column metal 11. Conversely, a decreased quantity of the platinum component of the tip 13 resides in portions 26, 27 corresponding to the upper and lower limits of the firing end 12 of the column metal 11. This is achieved in the same manner as before. In this instance, the greatest quantity of the platinum component of the tip 13 is approximately 86 % by weight, while the least quantity of the platinum component of the tip 13 is approximately 72 % by weight. This concludes that a difference between upper and lower limits of the platinum component is about 14 % by weight. Fig. 10 shows a fifth embodiment of the invention in which an iridium (Ir) or iridium-alloyed (Ir-Y₂O₃, Ir-La₂O₃, Ir-ZrO₂) layer 30 is secured to a front end surface 29a of a center electrode 29 by means of laser welding, electrical resistance welding or the like. In a spark plug 28 according to the fifth embodiment of the invention, a corrosive- and erosion-resistant platinum alloyed tip 33 is secured to a firing end 32 of a ground electrode 31 by means of the laser welding. (In similar manner to the previous embodiments) or by electrical resistance welding, e.g. into a recess, or by a combination of the two. This may be done before or after forming into an L-shape and attaching to the spark plug shell. The centre electrode 29 has a heat-conductive core 34 cladded by a column metal 36, while the ground electrode 31 has a heat-conductive core 35 cladded by a column metal 37. As apparent from the foregoing description, the noble metal component of the tip is changed depending on the magnitude of spark discharges to which the tip portion is subjected. Thus, the eroded layer of the tip is uniformly retained all through the firing end 12 of the column metal 11 when the spark plug exhausts its service life. This enables to reduce the quantity of the expensive platinum component used to the tip portion to which the decreased incidence of the spark discharges is subjected without unadvantageously losing the spark erosion resistant property. This results in attaining a long service life of the spark plug with a minimum use of the expensive platinum, thus significantly curtailing a manufacturing cost when reduced to mass production in industrial application. It is noted that the tip may be made of gold, palladium, iridium, rhodium or the like instead of the platinum metal used to the noble metal tip 13 according to the embodiments of the invention. It is also noted that the ground electrode 4 may be discretely prepared with its spark erosion taken into consideration instead of making it in integral with metallic shell 3 in the first embodiment of the invention. It is observed that the noble metal tip may be used to a firing end of the ground electrode in the first through fourth embodiment of the invention. It is also observed that instead of the noble metal layer 30, the noble metal tip 33 may be used to the front end surface 29a of the center electrode 29. It is appreciated that the platinum component of the tip 13 may be altered in its axial direction according to a spark erosion pattern predetermined on an experimental test result in which the noble metal tip is actually eroded in the first, second and fourth embodiment of the invention. It is also appreciated that the platinum component of the tip 13 welded to the ground electrode may be altered in its radial direction according to the spark erosion of a firing portion of the ground electrode. Further, it is observed that a noble tip may be previously made in which the platinum component is altered in its axial direction depending on the magnitude of the spark discharges to which the tip portion is subjected, and thereafter the tip may be secured to the firing end 12 of the column metal 11 of the center electrode 5 by means of the electrical resistance welding or the like. While the invention has been described with reference to the specific embodiments, it is understood that this description is not to be construed in a limiting sense in as much as various modifications and additions to the specific embodiments may be made by skilled artisan without departing from the scope of the invention as defined in the attached claims.

Claims: A spark plug electrode having an alloyed tip (13) secured to a firing end (14) of at least one of the opposed electrodes to form a spark gap between the opposing surface of said tip and opposed electrode respectively, the tip (13) including a spark-erosion resistant noble metal, the noble metal content of the tip varying in an axial direction of said one electrode (6), characterised in that the noble metal content varies in a direction parallel to said opposing surface of the tip in dependency on the amount of sparking to which that portion of the tip (13) is to be subject such that a largest weight percentage of the noble metal content is provided at portions of the tip subject to a relatively high amount of sparking, and wherein a smallest weight percentage of the noble metal content is provided at portions of the tip subject to a relatively low amount of sparking. A spark plug electrode according to claim 1, in which the noble metal content varies in dependency on a quantity of spark erosion to which that portion of the tip (13) is to be subject. A spark plug electrode according to claim 1, in which the noble metal content varies in dependency on the magnitude of spark discharges to which that portion of the tip (13) is to be subject. A spark plug electrode according to claim 1, 2 or 3, wherein the difference between largest and smallest weight percentage of the noble metal content of the tip is 10% by weight or more. A spark plug electrode according to claim 1, 2, 3 or 4, wherein the tip (13) comprises a platinum or platinum-based alloy containing nickel and iridium, iridium or iridium-based alloy containing Y₂O₃, La₂O, and ZrO₂. A spark plug electrode according to any one of the preceding claims, wherein the electrode (5) has a metal cladding (14) and a heat-conductive core (115) concentrically embedded in the cladding (14). A spark plug electrode according to claim 6, wherein the cladding metal (14) is made of a corrosion-and- erosion-resistant nickel alloy containing 15 wt% Cr and 8 wt% Fe, and the heat-conductive core (15) is made of silver or copper. A spark plug including an electrode according to any one of the preceding claims. A spark plug according to claim 8, wherein said electrode is a centre electrode of the spark plug. A spark plug according to claim 8 or 9, wherein said tip (13) is provided on a ground electrode.

Inventors: MATSUTANI WATARU, MUSASA MAMORU, MATSUTANI, WATARU, MUSASA, MAMORU, MATSUTANI, WATARU, C/O NGK SPARK PLUG CO., LTD., MUSASA, MAMORU, C/O NGK SPARK PLUG CO., LTD.
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1994
Main classification: H01T13
IPC codes: H01T13